1. Field of the Invention
The present invention relates generally to hydrocarbon fractionation apparatus and methods. In one aspect, the invention relates to apparatus and methods for determining the Reid Vapor Pressure of one or more product streams of a fractionating column. In another aspect, the present invention relates to apparatus and methods for controlling the Reid Vapor Pressure and quality of one or more product streams of a fractionating column.
2. Description of the Prior Art
Fractionation processes wherein a multi-component hydrocarbon stream is heated and fractionated into a bottoms product stream and one or more side draw or overhead product streams are commonly employed in refining and other similar applications. In carrying out such processes, it is important to maintain product quality while operating as efficiently as possible.
The Reid Vapor Pressure of a particular product stream is the equilibrium vapor pressure of the stream at 100.degree. F. (37.8.degree. C.). The Reid Vapor Pressure of a product stream is directly related to the composition of the stream. For example, the Reid Vapor Pressure of a bottoms product stream is a function of the concentration of the components therein. As a result, the Reid Vapor Pressure of a particular product stream can serve as an indication of the product quality of the stream.
Changes in certain process variables associated with a fractionating column such as the feed stream flow rate, the feed stream composition, and the amount of heat provided to the column directly affect the quality and Reid Vapor Pressure of the product streams. The quality of a product stream can be controlled by detecting changes in the Reid Vapor Pressure of the stream and manipulating one or more process variables in response to the changes detected to maintain the actual Reid Vapor Pressure of the stream substantially equal to the Reid Vapor Pressure of the stream that corresponds to the quality of the stream desired.
In the past, an accurate determination of the Reid Vapor Pressure of a particular product stream typically required a complex analysis such as a chromatographic analysis of the stream. In a continuous fractionation process, it is difficult to perform such an analysis on a continuous or frequent basis. Even where it is possible to place a chromatographic or other type of analyzer on-stream, the cost and maintenance difficulties associated therewith often make it undesirable to do so.
In order to alleviate the problem, apparatus and methods have been developed for determining the actual Reid Vapor Pressure of a product stream in the absence of a chromatographic or other type of complex analysis thereof. In U.S. Pat. Nos. 4,578,151 and 4,667,508, apparatus and methods for determining the actual Reid Vapor Pressure of a mixture of hydrocarbons in response to the actual temperature and vapor pressure of the mixture are disclosed. Vapor pressure charts are used with the temperature and vapor pressure readings to make the determination.
Although apparatus and methods for determining the Reid Vapor Pressure of a product stream in response to the temperature and vapor pressure of the stream are a solution to the problem, they do not provide an accurate Reid Vapor Pressure indication in all applications. Such apparatus and methods do not provide an accurate indication of the Reid Vapor Pressure of a product stream when the Reid Vapor Pressure of the stream varies over a wide range as the fractionation process is carried out.
Thus, there is a need for improved apparatus and methods for determining the Reid Vapor Pressure of a product stream in the absence of a chromatographic or other complex analysis thereof. There is a need for such apparatus and methods that can provide an accurate indication of the actual Reid Vapor Pressure of a product stream even when the actual Reid Vapor Pressure of the stream varies over a wide range as the fractionation process is carried out.